Query Helper append to existing field.

Hi! With CodeIgniter 3, I would like to append a string to an existing field. I did something like :

PHP Code:

$this->db->where('id', $id);
$this->db->set('field', "CONCAT(field, '" . $string."')", false);
$this->db->update('mytable'); 


I don't really like "not escaping" the user input. What is the right and secure way to append a string to an existing field ?